Iran's supreme leader said yesterday warned that the nation "will not yield to pressure" over a disputed presidential election, amid soaring tensions between Tehran and the West.
Ayatollah Ali Khamenei again said the result would stand, despite protests in which several people have died.
The main protest leader, Mir Hossein Mousavi, has not been seen in public for days, but his website quoted his wife saying protests would continue.
Protesters were again trying to gather in central Tehran. AFP reported that a large presence of Iranian riot police and a volunteer militia deterred demonstrators who were trying to gather near parliament in Tehran yesterday.
Police armed with batons and shields called on the few hundred people who had attempted to gather in Baharastan square to disperse, the witness said.
Since the June 12 presidential election results were announced, Tehran has been rocked by violent protests which have killed at least 17 people, but there has been no major demonstration or violence reported since Monday.
"No permission has been given for any rally," the interior ministry said in a statement earlier on Wednesday.
The witness said people were walking in groups near the square but were not shouting slogans or carrying any banners.
"In the recent incidents concerning the election, I have been insisting on the implementation of the law and I will be (insisting). Neither the system, nor the people will back down under force," Ayatollah Ali Khamenei said.
It was the latest indication that the clerical regime will not brook dissent over the re-election of President Mahmoud Ahmadinejad despite a wave of public demonstrations and complaints that the June 12 vote was rigged.
And in a sign security forces are wasting no time to put down protests, a large presence of riot police and Islamist Basij militiamen stopped a crowd of several hundred people trying to gather outside parliament in Tehran, a witness said.
In the latest diplomatic backlash over what Iran has branded Western meddling, Foreign Minister Manouchehr Mottaki said Tehran is considering whether to downgrade ties with Britain.
His comments came after the two governments expelled diplomats in a tit-for-tat move, with Tehran increasingly pointing the finger at London over the street violence that erupted in the aftermath of the election.
Tehran has accused Britain -- described by Khameini as the "most evil" of Iran's enemies -- of plotting against the election and seeking to stabilise the country.
It has expelled the BBC correspondent in Tehran and arrested a British-Greek journalist working for a US newspaper, one of at least two foreign reporters detained by the authorities.
Iran's interior minister also took aim at the United States, saying rioters were being funded by the CIA and the exiled opposition group the People's Mujahedeen.
US President Barack Obama, in his strongest comments yet, on Tuesday raised questions about the legitimacy of the election and expressed outrage over the violence against on opposition protesters.
Iran has refused to overturn the results of the poll but Khamenei -- who has ruled over the Islamic republic for 20 years -- has extended by five days a Wednesday deadline to examine vote complaints.
The authorities have also intensified a crackdown on opposition leader and defeated presidential candidate Mir Hossein Mousavi, with the arrest of staff at his newspaper and vitriolic attacks from the hardline press.
Another defeated candidate, former Revolutionary Guards chief Mohsen Rezai, has withdrawn his protest about election irregularities, in a blow to the opposition.
"(Iran's) political, social and security situation has entered a sensitive and decisive phase, which is more important than the election," Rezai said in a letter to the Guardians Council, the top election body.
Mousavi, who was premier in the post-revolution era, has urged supporters to keep demonstrating but to use "self-restraint" to avoid further bloodshed while another defeated candidate Mehdi Karroubi called for a mourning ceremony on Thursday for slain protesters.
Police have arrested 25 journalists and other staff at Mousavi's Kalemeh Sabz (Green Word) newspaper -- which was shut down about 10 days ago -- one of its editors said.
Intelligence police said they had found "evidence of a plot against national security" at a candidate's campaign office used for "psychological warfare," the official IRNA news agency said, without identifying the candidate.
The Revolutionary Guards, the elite force set up to protect the Islamic republic, has warned of a "decisive and revolutionary" riposte to any further protests.
The last opposition rally on Monday was crushed by hundreds of riot police armed with steel clubs and firing tear gas.
The foreign media is banned from reporting from the streets under tight restrictions imposed since the unrest was unleashed, but images of police brutality have spread worldwide via amateur video over the Internet.